Optical discs are media used to store a wide variety of digitally encoded data. Such discs are usually portable in nature and can be played in a variety of settings (personal computers, car audio players, home theater systems, handheld personal entertainment devices, home gaming systems, etc.).
A typical optical disc comprises a circular disc having one or more recording layers of light reflective material embedded in a refractive substrate. Each recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands along a continuously extending spiral track. A data transducing head uses a laser or similar light source to output a readback signal based on the different reflectivities of the pit and land areas. Decoding circuitry decodes the user data for output by the appropriate playback device.
An optical disc typically has a lead-in area, a program area and then a lead-out area. Table of contents or similar information are typically stored in the lead-in area to allow the readback system to identify the contents of the playback area. Multi-layer discs, popular for certain types of formats such as DVD-9, use multiple embedded, semi-translucent layers that can be accessed by switching the focal length of the readback system.
In a multi-layer disc, the readback system typically moves the transducing head radially across the disc (such as from inner diameter, ID to outer diameter, OD). When the first layer is completed, the readback system switches the focal length of the head and continues reading the next layer in the disc back across the radius of the disc (such as from OD to ID). The first layer has a lead-in area that stores a table of contents identifying the program area contents of all of the layers in the disc. The end of the final layer has a lead out area to indicate playback is complete.
Since the content information for an entire multi-layer disc is presently only stored on the first layer (in the lead-in area), there is generally no effective way to verify the contents of a second layer absent the first layer. This makes individual testing of layers difficult, and also makes it harder to ensure that the correct layers are assembled into the final replicated disc.